1. Field
This invention pertains to improvements in ferromagnetic bandpass filters such as YIG filters, and more particularly, to improvements in the response of such filters made possible through the use of integral compensating impedance transformers.
2. Prior Art
An ideal multi-stage YIG bandpass filter exhibits a constant bandwidth and passband shape over its entire range. Achieving this ideal requires constant coupling between filter resonators as well as constant coupling to the input and output resonators over the desired tuned range of the filter. Where constant couplings cannot be achieved, it would then be desirable as an alternative to provide couplings which change at the same rate in order to obtain a constant passband shape and a low equal ripple passband VSWR. Neither of these alternative sets of conditions has been achieved previously over multi-octave tuning ranges by ferromagnetic devices which incorporate loop coupling.
The bandwidth of the input and output resonator of a multi-section YIG filter as well as the bandwidth of a single stage YIG filter vary with tuning frequency. These changes in bandwidth are dependent upon the coupling loop reactance and upon variation of RF current along the length of the coupling loop, both of which change with frequency. FIG. 1 is a graph which provides an example of the change in the input stage bandwidth (BW.sub.e) 101 and also the adjacent resonator peak to peak bandwidth .DELTA.fp 102 as a function of resonator frequency. In FIG. 1, the ordinate 104 represents the bandwidth in megahertz while the abscissa 103 represents the center frequency in GHz. Curve 101 is plotted for a typical three-quarter turn loop while curve 102 is plotted for a typical half-turn loop.
The input stage bandwidth variation causes the filter passband VSWR and insertion loss ripple to change substantially over broad multi-octave tuning ranges. In addition, it can be seen from FIG. 1 that the adjacent resonator bandwidth obtained with typical loop structures produce relatively constant filter bandwidth at low RF frequencies, whereas the input bandwidth varies rapidly. At the higher frequencies, the adjacent resonator bandwidth increases substantially with frequency, whereas the input bandwidth decreases with frequency. In general, decreased input and output stage bandwidth causes increased passband ripple. Consequently, prior art multi-octave tuning range filters have a relatively large passband ripple over an appreciable part of their tuning range.
A commonly used approach for decreasing the filter passband ripple in prior art devices is to choose coupling loops with minimum bandpass variation over the frequency range of interest. A second commonly used approach is to degrade the unloaded Q of the resonators and thereby cause a substantial increase in filter insertion loss and concomitant rounding of the passband shape.
There are two prior art U.S. patents relating to this area of tunable filter, but neither discloses a means of providing constant bandwidths over wide frequency ranges. Although the first of these, U.S. Pat. No. 3,435,385, illustrates the use of variable capacitors in the form of varactor diodes to vary coupling in YIG filters, it does not disclose impedance transforming sections designed to achieve constant bandwidth.
The second of these prior art patents, U.S. Pat. No. 3,400,343, claims a first stripline feed to the first input resonator and a second stripline feed to the last output resonator, however, it does not disclose the use of stripline as an input compensating transformer. Additional background information for the present invention may be found in my prior U.S. Pat. Nos. 4,247,837 and 3,562,651.